The present invention relates to a method for bonding the electrodes in solid oxide fuel cell (“SOFC”) stacks to the interconnect layers in such stacks and, more particularly, to an improved method and materials for assembling solid oxide fuel cells to form a permanent, thermally and chemically stable bond between the electrodes and interconnects with good electrical contact between the electrodes and interconnect components. The present invention also relates to the specific compositions used in the bonding method.
A fuel cell is an energy conversion device that electrochemically reacts a fuel with an oxidant to generate a direct current. In a typical fuel cell, the cathode material provides the reaction surface for the oxidant and the anode material provides the reaction surface for the fuel. The electrolyte separates the oxidant and the fuel and allows for ionic transport of the reactant. The fuel and oxidant fluids (typically gases) are continuously passed through separate cell passageways with the discharges from the fuel cell removing the reaction products and heat generated in the cell.
Solid oxide fuel cells (“SOFC”) typically are formed by stacking a series of planar interconnect layers (often called “plates” or “interconnects”) and the fuel cells to form an integral fuel cell stack assembly. Most solid oxide fuel cell designs thus include two basic subassemblies, namely the fuel cells and related interconnect plates, stacked on top of one another in alternating fashion. These basic components of the stack must be assembled such that they remain together with good electrical contact at all times in order to reduce ohmic losses. Gaskets/seals positioned between each of the interconnect plates and cells help to prevent the undesirable leakage of the gases used by the fuel cells. Normally, a constant clamping force must also be exerted on the stack to ensure proper sealing and electrical contact during operation.
Currently, two basic stack constructions are used for SOFCs, i.e., planar cell stacks and tubular cell stacks/bundles. In both designs, the mechanical integrity of the stack and electrical contact between the fuel cells and interconnect subassemblies typically occurs through direct mechanical compression. In some cases, the stack components have also been “glued” together using sealing materials such as high temperature glasses and cements at the cell edges. The electrical contact between finished surfaces has also been improved somewhat by using a flexible/ductile conductive felt material, such as carbon paper in the case of proton exchange membrane fuel cells or metal foam/mesh/felts in the case of conventional SOFC stacks. However, heretofore the physical bond strength and electrical conductivity between electrodes and interconnects has been a significant limiting design problem with SOFCs.
Although the use of mechanical compression in fuel cell assemblies is a well-known technique for maintaining good mechanical integrity and electrical contact, a number of significant design issues still remain with such assemblies. For example, in order to ensure good electrical contact, a considerable amount of compression is normally required. However, excessive compressive forces are often undesirable in SOFC assemblies due to the inherent brittleness of certain ceramic components and the low dimensional tolerances associated with certain individual stack components.
In addition, the known compression mechanisms are often bulky and heavy due to the plurality of bolts/rods and thick end plates required to effectuate uniform compression. The use of such bulky loading/compression mechanisms adds cost and weight to the stacks, as well as a significant thermal mass.
Most conventional tie rod and bellows materials also tend to lose strength and creep over time under typical SOFC operating temperatures such as 600-1,000° C. That is, the compression load may ultimately become compromised due to changes and differences in mechanical properties of the tie rods and bellows at elevated operating temperatures. As a result, thermal expansion differences between the stack components and compression mechanisms must be carefully designed and monitored. Otherwise, the compression forces could vary during the thermal cycling and result in the loss of mechanical and electrical integrity of the stack.
In some prior art SOFC designs, bonding agents comprising conductive powders have been used to provide electrical connection between the cells and interconnect subassemblies. However, the adhesion between the subassemblies using such bonding agents is known to be poor, primarily because insufficient bonding strength develops between the two components when the stacks operate at temperatures between 600 and 1,000° C.
In addition, an excessively strong adhesive bond between fuel cells and interconnects with different thermal expansion properties can generate unwanted thermal stresses and cause delamination, deformation, or even cell fracture during thermal cycling. Obviously, poor adhesion and delamination during thermal cycling inevitably have detrimental effects on the overall stack integrity and the electrochemical performance of the stack. Thus, a proper bonding method with thermally compatible materials is essential to providing an effective electrical conduction path while at the same time ensuring adequate bond strength to meet the combined electrical, thermal, and mechanical needs of the SOFC stack. Various approaches to improving the electrical contacts and bonding materials and their functionality have been attempted in the past. For example:
U.S. Pat. No. 6,703,154 discloses the use of a spring-loaded compression bellows in solid oxide fuel cell stacks to reduce the thermal stress caused by direct mechanical compression. The compression bellows improve the stack's tolerance to thermal cycling. However, one obvious problem with this approach concerns the selection of suitable high temperature resistance alloys for the bellow shells since most alloys tend to creep and lose stiffness at high temperatures.
U.S. Patent application 20040101742 discloses a current collector comprising an electrically conductive mesh spacer between each electrode and its adjacent interconnect. In order to maintain good electrical contact under compression, a compliant spacer or buffer layer such as metal felt, flexible mesh or metal foam must be placed between the current collectors and cells. The mesh spacer is secured to the interconnect plate through brazing or welding and the mesh is resilient to maintain mechanical and electrical contact with the electrodes and interconnect plates during assembly and operation. Although contact can be improved with such current collectors, an excessive compression force is still necessary to maintain good electrical contact between the current collectors and electrodes.
U.S. Pat. No. 5,922,486 discloses a method for joining multiple solid oxide fuel cell units using co-firing. “Buffer” layers are interposed between each of the electrodes and the interconnect layer to improve the conductivity and bonding. The buffer layers joining the interconnect layers and electrodes (both the cathode and anode) comprise either CuO+NiO+La0.8Sr0.2MO3 or CeO2+NiO+La0.8Sr0.2MnO3. Since La0.8Sr0.2MO3 tends to decompose under a reducing atmosphere (anode side) and the conductivity of NiO is limited under an oxidation atmosphere (cathode side), the bonding strength and electrical properties can be compromised using this system. Also, the disclosed process is limited to ceramic interconnects which comprise either La0.7Ca0.3CrO3 or La0.8Sr0.2CrO3+CaCO3 and require a higher co-firing temperature (typically 1,275° C.) in order to achieve an effective bond.
U.S. Pat. No. 5,290,642 (Minh et al., GE, 1994) teaches an assembly and bonding method for monolithic solid oxide fuel cells in which the bonding agents are formed by mixing a powder of anode materials for the anode bonding agent or a cathode ceramic powder for the cathode bonding agent. The agents are applied in order to wet the densified monolithic structure components and then heat treated in a furnace or microwave to sinter and densify the bonding agent. Again, one distinct disadvantage of this prior art technique is that it bonds the ceramic interconnect layer to the cathode and anode at relatively high temperatures.
U.S. Pat. No. 5,702,823 teaches a method for producing anode bonding materials for anode-to-anode bonding and anode-to-interconnect bonding in solid oxide fuel cells. The anode/interconnect bonding material includes powders of reactive ingredients with nickel oxide, zirconium oxide, cobalt oxide, calcium oxide or strontium oxide as the major components. The reactive ingredients are selected from tungsten, tantalum, niobium, molybdenum and titanium. Ceramic powders are mixed with organic binders and solvents to form a bonding slurry that can then be brushed or sprayed onto the surfaces of the anode and interconnect pairs to be bonded. The bonding operation takes place at temperatures between 1,000-1,300° C. and the bond materials react with the anode and/or interconnect materials to provide the requisite bond. Again, the bonding temperature is too high for typical stack assembly operation which generally falls in the range of 600 to 1,000° C.
Various prior art electrically conductive coatings have also been proposed to improve the surface of current collectors, i.e., to reduce contact resistance including, for example, Ag—La0.8Sr0.2CrO3 coatings (see C. Hatchwell et al., Journal of Power Sources, p. 64, 1999), or spinel and perovskite coatings (see Y. Larring et al., J. Electrochem. Soc., p. 3251, 2000), and LaNi0.6Fe0.4O3 (see R. Basu et al., J. Solid State Electrochem, p. 416, 2003).
In the past, active brazing has also been proposed to join the ceramic cells to metallic interconnect layers. However, the brazing process itself has proven to be difficult and less effective due to the materials instability and potential interactions between the active brazing materials and electrodes under normal processing and operating conditions.